U.S. patent number 6,278,051 [Application Number 09/529,149] was granted by the patent office on 2001-08-21 for differential thermopile heat flux transducer.
This patent grant is currently assigned to Vatell Corporation. Invention is credited to Hume L. Peabody.
United States Patent |
6,278,051 |
Peabody |
August 21, 2001 |
Differential thermopile heat flux transducer
Abstract
A thin sensor for heat flux and temperature, designed for
adhesive attachment to a surface, is manufactured on a flexible
insulated metallic substrate. The sensor exhibits a combination of
high sensitivity for heat flux and low resistance to the flow of
heat. These characteristics enable it to measure heat flux at
surface boundaries with improved accuracy over conventional heat
flux transducers because the temperature drop produced by the
sensor is very small. The response by the sensor to radiation,
convection and conduction are equal. As such, the sensor can be
calibrated in one mode of heat transfer and used for measurement in
other modes. The high sensitivity of the sensor makes it ideal for
measuring heat flow through insulating materials, and well adapted
to instrumenting heat flow in buildings, detecting fires at an
early stage, or remotely measuring the temperature of string and
web products in industrial processing.
Inventors: |
Peabody; Hume L. (Laurel,
MD) |
Assignee: |
Vatell Corporation
(Christiansburg, VA)
|
Family
ID: |
24108723 |
Appl.
No.: |
09/529,149 |
Filed: |
April 7, 2000 |
PCT
Filed: |
October 09, 1997 |
PCT No.: |
PCT/US97/18333 |
371
Date: |
April 07, 2000 |
102(e)
Date: |
April 07, 2000 |
PCT
Pub. No.: |
WO99/19702 |
PCT
Pub. Date: |
April 22, 1999 |
Current U.S.
Class: |
136/225; 136/227;
374/179; 374/30; 374/E17.015 |
Current CPC
Class: |
G01K
17/20 (20130101); H01L 35/00 (20130101); H01L
35/32 (20130101); H01L 35/34 (20130101) |
Current International
Class: |
G01K
17/20 (20060101); G01K 17/00 (20060101); H01L
35/32 (20060101); H01L 35/34 (20060101); H01L
35/00 (20060101); H01L 035/28 () |
Field of
Search: |
;136/225,227
;374/30,179 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Gorgos; Kathryn
Assistant Examiner: Parsons; Thomas H
Attorney, Agent or Firm: Morgan & Finnegan LLP
Claims
What is claimed is:
1. A differential thermopile heat flux sensor, comprising:
(a) an electrically insulating substrate made of a material which
has high thermal conductivity;
(b) a first layer disposed above said electrically insulating
substrate, said first layer comprising (1) insulating material and
(2) a plurality of thin raised metal pads covering a part of the
surface of said substrate;
(c) a second layer disposed above said first layer, said second
layer comprising (1) insulating material and (2) a plurality of
metal links;
(d) a third layer disposed above said second layer, said third
layer comprising (1) insulating material and (2) a plurality of
metal links interconnecting said links of said second layer;
(e) a fourth layer disposed above said third layer, said fourth
layer comprising (1) insulating material and (2) a plurality of
metal pads; and
(f) a top layer disposed above said fourth layer made of a thin
layer of high thermal conductivity material.
2. The differential thermopile heat flux sensor of claim 1 wherein
said links of said second layer each contain a first end and a
second end, said first ends disposed in contact with said pads of
said first layer.
3. The differential thermopile heat flux sensor of claim 2 wherein
said links of said third layer each contain a first end and a
second end, said first ends of said third layer disposed in contact
with said second ends of said second layer, and said second ends of
said third layer disposed in contact with said first ends of said
second layer.
4. The differential thermopile heat flux sensor of claim 3 wherein
said pads of said fourth layer are disposed in contact with said
first ends of said third layer.
5. The differential thermopile heat flux sensor of claim 1 wherein
said pads of said first layer and said links of said second layer
are made of one metal, and said links of said third layer and said
pads of said fourth layer are made of a second metal.
6. The differential thermopile heat flux sensor of claim 5 wherein
said first metal is nickel and said second metal is copper.
7. The differential thermopile heat flux sensor of claim 1 wherein
said substrate is formed from a metal by coating one of its
surfaces with a thin layer of an electrically insulating
material.
8. The differential thermopile heat flux sensor of claim 1 wherein
said substrate is formed of aluminum and is electrically insulated
by an anodic oxidation process.
9. The differential thermopile heat flux sensor of claim 1 wherein
said substrate is a flexible sheet.
10. A heat flux sensor comprising:
(a) a flexible aluminum substrate;
(b) a thin layer of an electrically insulating material disposed on
at least one side of said flexible aluminum substrate;
(c) a plurality of nickel pads disposed on said flexible aluminum
substrate, and polymeric insulating material disposed between said
nickel pads;
(d) a plurality of nickel links, each of said nickel links
comprising a first end and a second end, and polymeric insulating
material disposed between said nickel links, each of said first
ends disposed above and in contact with one of said nickel
pads;
(e) a plurality of copper links, and polymeric insulating material
disposed between said copper links, each of said copper links
comprising a first end and a second end, each of said first ends of
said copper links disposed above and in contact with a second end
of one of said nickel pads, each of said second ends of said copper
links disposed above and in contact with a first end of another
nickel pad, thereby linking said nickel pads;
(f) a plurality of copper pads, and polymeric insulating material
disposed between said copper pads, each of said copper pads
disposed above and in contact with a first end of one of said
copper pads; and
(g) a layer of high thermal conductivity material disposed above
and in contact with said plurality of copper pads.
11. A heat flux sensor comprising:
(a) a substrate having a thin layer of an electrically insulating
material disposed on at least one side;
(b) a thin layer of high thermal conductivity material spaced apart
from said substrate;
(c) a plurality of first metal electrodes disposed between said
substrate and said thin layer of high conductivity material;
(d) a plurality of second metal electrodes disposed between said
substrate and said thin layer of high conductivity material, each
of said second metal electrodes disposed in contact with two of
said first metal electrodes, wherein said first metal electrodes
are made of a material different from said second metal
electrodes;
(e) polymeric insulating material disposed between said plurality
of first metal electrodes, said second metal electrodes, said
substrate and said thin layer of high conductivity material;
(f) first metal pads, said first metal pads disposed between said
substrate and said plurality of first metal electrodes; and
(g) second metal pads, said second metal pads disposed between said
plurality of second metal electrodes and said thin layer of high
conductivity material.
12. The heat flux sensor of claim 11 wherein said plurality of
first metal electrodes and said first metal pads are made of the
same material.
13. The heat flux sensor of claim 11 wherein said plurality of
second metal electrodes and said second metal pads are made of the
same material.
14. A heat flux sensor comprising:
(a) substrate means for electrically insulating at least one side
of said heat flux sensor;
(b) electrode means for conducting electricity within said heat
flux sensor, said electrode means comprising a plurality of hot and
cold junctions of electrodes;
(c) polymeric insulating means for insulating said electrode
means;
(d) covering means for preventing development of hot locations
above the polymeric insulating means;
(e) metallic means for conducting heat from cold junctions of said
electrode means to said substrate means; and
(f) metallic means for conducting heat to hot junctions of said
electrode means from said covering means.
15. The heat flux sensor of claim 14 wherein said substrate means
for electrically insulating comprises a flexible substrate having a
thin layer of an electrically insulating material disposed on at
least one side.
16. The heat flux sensor of claim 14 wherein said electrode means
comprises a plurality of first metal electrodes and a plurality of
second metal electrodes, each of said second metal electrodes
disposed in contact with two of said first metal electrodes,
wherein said first metal electrodes are made of a material
different from said second metal electrodes.
17. The heat flux sensor of claim 16 wherein said polymeric
insulating means comprises polymeric insulating material disposed
between said plurality of first metal electrodes, said second metal
electrodes, said substrate means and said means thin layer of high
conductivity material.
18. The heat flux sensor of claim 14 wherein said covering means is
a thin layer of high conductivity material.
19. The heat flux sensor of claim 14 wherein said metallic means
for conducting heat from cold junctions of said electrode means to
said substrate means comprises a plurality of metal pads.
20. The heat flux sensor of claim 14 wherein said metallic means
for conducting heat to hot junctions of said electrode means from
said covering means comprises a plurality of metal pads.
21. A differential thermopile heat flux sensor, comprising:
(a) an electrically insulating substrate made of a material which
has high thermal conductivity;
(b) a first layer disposed above said electrically insulating
substrate, said first layer comprising (1) a plurality of first
metal bridges made of a first metal and (2) insulating material
disposed between said plurality of first metal bridges;
(c) a second layer disposed above said first layer, said second
layer comprising (1) a plurality of first metal links made of said
first metal, (2) a plurality of second metal links made of a second
metal and (3) insulating material disposed between said plurality
of first metal links and said second metal links; and
(d) a third layer disposed above said second layer, said third
layer comprising (1) a plurality of second metal bridges made of
said second metal and (2) insulating material disposed between said
said plurality of second metal bridges;
wherein each of said first metal bridges is in contact with one of
said first metal links and one of said second metal links, and
wherein each of said second metal bridges is in contact with one of
said first metal links and one of said second metal links.
22. The differential thermopile heat flux sensor of claim 21
further comprising:
(e) a top layer disposed above said third layer made of a thin
layer of high thermal conductivity material.
23. The differential thermopile heat flux sensor of claim 21
wherein said first metal is nickel.
24. The differential thermopile heat flux sensor of claim 21
wherein said second metal is copper.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related generally to heat flux transducers
and, more particularly, to a differential thermopile heat flux
transducer having a plurality of layers and a high thermal
conductivity top coating which enables the transducer to measure
heat flux with improved accuracy.
2. Description of the Related Art
Heat flux sensors are routinely used to measure the rate and
direction of heat energy flow. For example, heat flux sensors (or
"transducers") have been used in building energy management
applications since the 1950's. Methods for using heat flux
transducers to evaluate thermal performance of building materials
are generally well understood. Heat flux transducers designed for
surface mounting are inexpensive and easy to install. However, it
is often difficult to acquire accurate, useful data with heat flux
sensors.
In combination with a temperature sensor disposed at the same
location, a heat flux sensor and a temperature sensor can be used
to measure temperature, heat flux, the heat transfer coefficient,
the effective thermal capacity, the projected rate of temperature
change at the present heat flux, and the projected rate of
temperature change at any other value of heat flux. In addition,
the temperature and heat flux signals can be compared to detect
drift or failure of either sensor.
Measurement of heat flux is critical to the understanding and
control of many thermal systems. When both heat flux and
temperature data are available for the same point on a surface,
material properties such as thermal resistance and thermal
diffusivity can be calculated. Heat flux measurement is essential
for the performance evaluation of insulative building materials.
This is because it is often difficult or impossible to predict the
installed performance of insulative materials from laboratory
experiments.
The heat flux through a surface cannot, however, be measured
without some disturbance caused by insertion of the measuring
device into the path of heat flow. The amount of change produced by
the measuring device depends on many factors. These include: the
contact resistance between the heat flux transducer and the test
wall, as well as other physical parameters such as surface
emissivity, surface roughness and the thermal resistance of the
heat flux transducer itself. These factors include the effective
series thermal resistance, R.sub.m. The following relationship is
described in Trethowen, H. A., "Systematic Errors with
Surface-Mounted Heat Flux Transducers and How to Live with Them",
In-Situ Heat Flux Measurements in Buildings--Applications and
Interpretations of Results, CRREL Special Report 91-3, 1991, U.S.
Army Cold Regions Research and Engineering Laboratory, Hanover,
N.H., Hanover, N.H., pp. 15-27:
Where:
R.sub.m =effective series thermal resistance
R.sub.h =series thermal resistance (conductive) of the heat flux
transducer alone
R.sub.c =thermal contact resistance between the heat flux
transducer and substrate
R.sub.ms =total thermal surface resistance (convective and
radiative) over the heat flux transducer
R.sub.s =total thermal resistance (convective and radiative) over
surrounding area
The effective series resistance of a surface mounted heat flux
transducer is the most important single factor affecting the error
produced by disturbance of the measured heat flux. If R.sub.ms and
R.sub.s are made approximately equal by matching the emissivity and
the surface roughness of the heat flux transducer to corresponding
values for the surrounding material, the effective series
resistance is reduced to the sum of the heat flux transducer series
thermal resistance and the thermal contact resistance. The series
thermal resistance can vary widely in commercially available
surface mounted heat flux transducer's: from about 0.002
m.sup.2.degree. C./W to 0.1 m.sup.2.degree.C./W. The thermal
contact resistance is minimized by attaching the sensor to the
surface with a very thin layer of high thermal conductivity
adhesive.
For maximum utility in building energy management, a heat flux
sensor should have a series thermal resistance of less than
1.times.10.sup.-4 m.sup.2.degree. C./W. However, when the series
thermal resistance of a thermopile type heat flux transducer is
very low, its sensitivity may also be low because the low thermal
resistance only produces a small temperature difference.
Another factor which affects the utility of a heat flux sensor in
building energy management is shunting of heat flux around the
sensor. A common way of solving this problem is to use a sensor
with a large area, in the belief that the long path around the
sensor will reduce the effects of shunting. This solution is not
effective when the sensor has high thermal resistance, because the
measured heat flux is that which passes through the sensor, and it
is reduced by the sensor's thermal resistance. A more effective
solution is to employ a sensor with low thermal resistance.
Large area sensors are also commonly used to measure heat flux over
non-homogeneous areas, such as across wall studs in framed
buildings, or on truss roofs. Unless the areas of the sensor
covering the wall studs and the surrounding structure are in the
same proportion as in the entire structure, this practice
introduces an error. A better way of measuring heat flux over
non-homogeneous areas is to employ small heat flux sensors over
each representative part of the structure, and then calculate the
total heat flux for each part using its actual total area. The
total heat flux is then calculated by summing these amounts.
Another disadvantage of large area heat flux sensors is that they
are relatively expensive. According to a recent survey, the typical
cost of a commercially available 12" by 12" heat flux sensor is
over $600.
Copper conductors of printed circuit boards may be produced by a
number of processes. The most common of these is photoetching. In
this process, a board completely coated with copper and covered by
a photopolymer is (1) exposed to ultraviolet light through a
negative transparency of the desired conductor pattern, (2) solvent
washed to remove the polymer where it has not been hardened by
exposure and (3) acid etched to expose the desired conductors.
A second process for producing printed circuit boards, known as the
additive process, consists of a first step of electroless
deposition of a very thin nickel layer representing the desired
conductor pattern, followed by a second step of electrolytic
deposition of the desired thickness of copper on the nickel
conductors.
In a third process, which is less frequently used, conductors are
deposited as inks on an insulating substrate by screen printing.
The ink traces are dried to a solid by rapid heating in a vapor
reflow oven, then converted to metal by an elevated heat treatment.
This process could be adapted to heat flux sensor manufacturing, if
it could be used to deposit conductors of two different metals in
an appropriate pattern.
In U.S. Pat. No. 4,779,994, a thin film heat flux sensor and its
method of manufacture are disclosed. The manufacturing method has
certain drawbacks which limit the sensor performance and restrict
the range of applications. The manufacturing cost of the sensor is
high because it is made by multiple stages of sputtering through
shadow masks. Another drawback is the relatively low sensitivity of
the sensor. In U.S. Pat. No. 4,779,994, heat flux is measured by
measuring the temperature drop across a very small thermal
resistance, and signals are of the order of a few microvolts per
watt/cm.sup.2. In many applications for such sensors, heat flows to
be measured are a small fraction of 1 watt/cm.sup.2, and thin film
sensors cannot be used.
Prior heat flux sensors also suffer additional drawbacks. Prior
heat flux sensors fail to be equally responsive in sensing
radiation, convection and conduction heat transfer. Moreover, such
prior heat flux sensors are not responsive to heat transfer in both
directions through the sensor. In addition, such prior sensors
cannot be bent over a radius without loss of continuity or
calibration.
SUMMARY OF THE INVENTION
The present invention overcomes the aforementioned disadvantages by
providing a heat flux sensor (hereinafter called the "Episensor"),
which contains a thermopile, including an electrically insulating
substrate made of a material which has high thermal conductivity; a
first layer disposed above said electrically insulating substrate,
the first layer including (1) insulating material and (2) a
plurality of thin raised metal pads covering a part of the surface
of said substrate; a second layer disposed above said first layer,
the second layer including (1) insulating material and (2) a
plurality of metal links; a third layer disposed above the second
layer, the third layer including (1) insulating material and (2) a
plurality of metal links interconnecting the links of the second
layer; a fourth layer disposed above the third layer, the fourth
layer including (1) insulating material and (2) a plurality of
metal pads; and a top layer disposed above the fourth layer made of
a thin layer of high thermal conductivity material. Other aspects
of the invention will become more readily apparent upon a review of
the following detailed description of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an isometric view of a first generation elementary heat
flux sensor comprising a single thermocouple pair, formed on a thin
sheet of anodized aluminum.
FIG. 2 is an isometric view of an elementary heat flux sensor
comprising a single thermocouple pair shown together with the masks
used in its fabrication.
FIG. 3 shows an arrangement of many elementary heat flux sensors on
a thin sheet of anodized aluminum, which, together with a
thermocouple, comprise a heat flux transducer.
FIG. 4 is a side view of the elements of a second generation heat
flux sensor developed during the design of the preferred embodiment
of the present invention.
FIG. 5 is a side view of the elements of a third generation heat
flux sensor developed during the design of the preferred embodiment
of the present invention.
FIG. 6 is a side view of the elements of a fourth generation heat
flux sensor developed during the design of the preferred embodiment
of the present invention.
FIG. 7 is a side view of the elements of a fifth generation and
preferred embodiment of the heat flux sensor of the present
invention.
FIG. 8 depicts the effects of top layer thickness on the interface
temperature of the sensor.
FIGS. 9-13 depict various stages of one technique for manufacturing
the preferred embodiment of the present invention depicted in FIG.
8.
FIG. 9 is a perspective view of the anodized aluminum substrate
with a first layer of high thermal resistance polymer deposited
thereon.
FIG. 10 depicts four nickel pads, isolated in space, which make up
a portion of the first layer of the preferred embodiment of the
present invention depicted in FIG. 8.
FIG. 11 depicts four nickel links, which make up a portion of the
second layer of the preferred embodiment, as deposited above the
first layer depicted in FIG. 10.
FIG. 12 depicts four copper links, which make up a portion of the
third layer of the preferred embodiment, as deposited above the
second layer depicted in FIG. 11.
FIG. 13 depicts the four copper pads, which make up a portion of
the fourth layer of the preferred embodiment, as deposited above
the third layer depicted in FIG. 12.
DETAILED DESCRIPTION OF THE STRUCTURE AND DEVELOPMENT OF
THE PREFERRED EMBODIMENT OF THE PRESENT INVENTION
With reference to the attached drawings, the structure and
development of the preferred embodiment (the "Episensor") of the
present invention will now be described. Identical reference
numerals in different drawings refer to the identical structure.
The first number of each reference numeral indicates the first
figure in which that reference numeral has been introduced.
Development of the Episensor
1. The 1st Generation Improved Heat Flux Sensor
In U.S. Pat. No. 5,990,412, herein incorporated by reference, a
heat flux sensor and its method of manufacture, which contained
various improvements over prior known heat flux sensors and their
methods of manufacture, were disclosed. This first generation
improved heat flux sensor is depicted in FIG. 1. With reference to
FIG. 1, the sensor 100 consists of a thin sheet of aluminum 120
which has been anodized on at least one surface 130. Anodization
produces a thin aluminum oxide layer 140 to electrically insulate
the sensor elements from the aluminum. This layer is thin enough to
allow heat to flow through the sensor with little resistance. A
part of the anodized surface 130 of the sheet 120 is coated with a
polymeric insulating pad 150. This pad locally increases resistance
to the flow of heat over the portion of the surface it covers. A
copper signal electrode 160 is deposited partly on top of the
insulating pad 150 and partly on the aluminum oxide layer 140. A
copper reference electrode 170 is deposited entirely on the
aluminum oxide layer 140. One end of a nickel crossover electrode
185 overlaps the part of the copper signal electrode which is on
top of the insulating pad 150, forming a first thermocouple
junction 186. Theother end overlaps the copper reference electrode,
forming a second thermocouple junction 187. Certain steps in the
construction of this elementary heat flux sensor are depicted in
FIG. 2. The aluminum sheet 120 is prepared by anodization, a
process which exposes the metal surface to an oxidizing acid in an
electrolytic tank. The oxide layer 140 built up by this process is
dense and has very good electrical insulating qualities. After the
aluminum is anodized, the insulating pad 150 is deposited on its
insulated surface by silk-screening of an ink containing a polymer
dielectric. The ink is deposited on the surface through an aperture
209 in a first silk screen 210, and then dried and heat-cured to
produce the solid insulating pad 150. The next step is to apply a
copper-containing ink through apertures 211 and 212 in a second
silk screen 213, followed by drying and heat-curing. This step
produces the copper electrodes of thermocouples 160 and 170. The
final step of fabrication is to apply a nickel-containing ink
through aperture 214 in a third silk screen 215, followed by drying
and heat-curing. This completes the first generation heat flux
sensor. The copper electrodes of the two thermocouples are
connected to a voltage measuring instrument 280, providing a direct
indication of heat flux.
The first generation heat flux sensor operates as follows. When
heat flows through the substrate from its top to its bottom
surface, a difference in temperature develops between thermocouples
186 and 187. This occurs because the total resistance of the heat
path through the aluminum sheet and the polymeric insulating pad
150 is greater than that through the aluminum sheet alone. The
temperature difference will persist as long as the heat continues
to flow. The thermocouple 186 located on the insulating pad 150
will be slightly hotter than thermocouple 187. This difference in
temperature will produce a small positive voltage across the copper
electrodes of the sensor, and the voltage will be sensed by the
voltage measuring instrument 280. The magnitude of the voltage is
proportional to heat flux through the sensor because the difference
in the thermocouple temperature is proportional to heat flux.
If the heat flux direction is reversed, the temperature of the
thermocouple 186 will be lower than that of thermocouple 187, and
the polarity of the signal will be reversed. Thus the polarity of
the signal indicates the direction of heat flow.
The magnitude of the voltage signal produced by the first
generation heat flux sensor is very small. Typically the
temperature difference across the insulating pad 150 will be a
fraction of a degree centigrade, and the signal produced by the
opposed potentials of the two thermocouples will be a few
microvolts, not enough for accurate measurements in real
applications. To produce a more useful signal, a sensor may be
constructed of many elementary heat flux sensors, connected in a
series arrangement which adds their voltages together. Up to
thousands of thermocouple pairs may be so connected to measure heat
flux over a large area. Such an arrangement is shown in FIG. 3.
More particularly, FIG. 3 shows a heat flux transducer 316,
fabricated using dielectric and conductive inks. The heat flux
transducer is fabricated on a thin sheet of aluminum 120 which has
been anodized over its entire top surface to electrically isolate
the sensor elements. The heat flux sensor 317 comprises a series
connected array of 18,900 elementary heat flux sensors as shown in
FIG. 1. The ends of this array terminate in connection leads 318
and 319. An individual wire thermocouple 320 is bonded to the top
surface of the aluminum sheet 120, and its connection leads 321 and
322 are located between the leads 318 and 319 of the heat flux
sensor 317. Each of the four connection leads is connected to a
wire of the flat cable 323, which carries the signals of the heat
flux transducer to an amplifier 324.
In employing the heat flux transducer of FIG. 3 for measurements,
the surface whose heat flux and temperature are to be measured is
prepared with a thin layer of adhesive, such as Duro All-Purpose
Spray Adhesive, distributed by Loctite Corporation. The heat flux
transducer is then pressed flat onto the prepared surface with the
heat flux sensor 317 and thermocouple 320 side facing out. Then the
flat cable 323 is connected to the amplifier 324, and measurements
can begin.
The thick film conductors used in the sensor of the invention are
conductive copper and nickel inks. These inks, as well as methods
for their deposition and curing, were developed by Toronaga
Technology Inc.
2. The 2nd Generation Heat Flux Sensor
Although the first generation sensor has greatly improved
sensitivity and performance characteristics over prior sensors,
additional development has been conducted to further improve the
design. In particular, although the first generation sensor can be
adequately manufactured using the silk screening process depicted
in FIG. 2, the deposition of metal conductors which bridge the
difference in height between the substrate and the top of the
insulating pads has proven to be difficult. In addition, it has
been found that the sensitivity of the first generation sensor to
conducted heat flux is very different from its sensitivity to
radiated heat flux.
In response to these difficulties, the second generation heat
sensor depicted in FIG. 4 was developed. More particularly, FIG. 4
is a sectional view of the second generation heat sensor 400, which
is mounted on a substrate 405, and contains three layers 410, 420
and 430. Each of the three layers is approximately 0.001" (0.003
cm) thick. The bottom layer includes a plurality of nickel pads 410
surrounded by polymeric insulating material 415. The middle layer
has links of nickel 420 is overlapping the bottom layer pads 410,
each link 420 is surrounded by a narrow region of polymeric
insulating material 415. The top layer has links of copper 430,
each of which partially overlaps two of the nickel links 420, each
link 430 being surrounded by a narrow region of polymeric
insulating material 415. This arrangement forms a continuous
conductor with a regular sequence of Cu/Ni thermocouple pairs. One
of the junctions in each pair is located directly over a nickel pad
of the first layer. The second one is located over the polymeric
insulating material in the first layer.
This second generation heat sensor has the additional feature that
the top surface of the sensor is smooth. It was anticipated that
this would enhance heat transfer in the conductive mode by reducing
contact resistance, and make the resulting device a much more
reliable product.
This second generation heat flux sensor has been tested
extensively, and its performance was disappointing. Finite element
analysis showed the reason: only a small part of the temperature
difference produced across the sensor by passage of heat flux is
actually transduced into a voltage by the thermopile. Another
problem is that the sensor has different responses to radiative,
convective and conductive heat flux. In view of these problems,
further analysis and redevelopment of the sensor design was
conducted.
The sensitivity of a sensor covering a given area is proportional
to the density of thermocouple pairs that can be produced by the
ink deposition process. In one process for manufacturing heat flux
sensors, the minimum overlap of layers, minimum line widths and
minimum space between lines, are all limited to 0.005" (0.013
cm).
The electrical output of such a sensor is the sum-of the individual
small voltages produced by many Cu/Ni thermocouple pairs. One index
of the performance of such a sensor is the proportion of the
temperature difference produced by heat flux across the three
layers that is actually sensed by the thermocouple pairs. In actual
tests, the voltage produced by the second generation sensor was
less than 10% of that predicted by finite element analysis. Further
study of the second generation sensor through finite element models
of the configuration resulted in a conclusion that there are two
reasons for its poor performance. First, the stack of three layers
of metal, consisting of a bottom pad and middle link of nickel and
a top pad of copper, produce an approximately equal division of
temperatures across the device, and limit the temperature at the
"cold" junction to 1/3 of the temperature drop below the top
temperature. This temperature is not very different from that of
the other ("hot") junction, which is insulated by a layer of
polymer on the bottom. Second, heat flowing horizontally through
the nickel link in the middle layer tends to further equalize the
temperature of the two junctions and reduce the voltage output even
more.
One hundred sensors of this design were made, and the first 50
assembled had an average sensitivity of 26.41
millivolts/W/cm.sup.2, about one fourth of what had been projected.
Competitive sensor products provide about 60
millivolts/W/cm.sup.2.
In analyzing the deficiencies of this sensor, a finite element
model was generated to try to predict the output of the current
design. Assuming that the temperature at the cold junction
thermocouple is close to that of the substrate and the temperature
of the hot junction thermocouple is close to that of the top of the
insulation, a .DELTA.T across the insulation layer can be
calculated. This .DELTA.T should be proportional to the incident
heat flux to the system. The model predicted a .DELTA.T across the
insulation layer of 4.638.degree. C. at a heat flux of 10
watts/cm.sup.2.
From this, a sensitivity of a sensor can be theoretically
calculated according to the equation: ##EQU1##
The corresponding Seebek coefficient for the Cu/Ni ink thermocouple
is approximately 4.2 .mu.V/.degree. C. and the number of
thermocouple pairs (#TC pairs) is 18,900, resulting in a predicted
sensitivity of 36.921 mV/(W/cm.sup.2) for the second generation
design. This agrees fairly closely with the average experimental
value of 26.41 mV/(W/cm.sup.2).
Sensitivity can be improved in one of three ways: (1) increase the
number of thermocouple pairs, (2) change the Seebek coefficient, or
(3) raise the .DELTA.T across the insulation layer. The number of
thermocouple pairs was limited by the resolution of the fabrication
process, and the only metals that could be deposited by the process
were copper and nickel. Various changes were made in the model to
try to improve the .DELTA.T.
Upon analyzing the deficiencies of this second generation sensor,
the largest problem was determined to be the lateral flow of heat
along the links. Rather than heat conducting through the hot
junction, it is conducting laterally along the link, to the cold
junction, and then directly to the substrate. This results in a
lower .DELTA.T than would be provided if no "communication" existed
between the hot and the cold junction. This could account for a
lower sensitivity than what was predicted.
3. The 3rd Generation Heat Flux Sensor
With reference to FIG. 5, a third generation sensor with improved
sensitivity over the second generation sensor is shown. This
improved sensitivity is the result of insulating the "hot" junction
from the cold face of the sensor by a layer of polymer, and
insulating the "cold" junction from the hot face of the sensor by a
layer of polymer. With reference to FIG. 5, the third generation
sensor 500, as mounted on a substrate 505, contains a bottom layer
having nickel pads 510 surrounded by polymeric insulating material
515. The middle layer contains both nickel links 520 and copper
links 525, each of which are surrounded by polymeric insulating
material 515. The top layer contains copper pads 530 surrounded by
polymeric insulating material 515. Nickel pads 510 and copper pads
530 serve as bridges which interconnect nickel links 520 and copper
links 525. For example, one nickel pad 510 bridges a nickel link
520 with a copper link 525.
Unfortunately, this design requires that the center layer of the
sensor contain both metals. Because of process limitations, such an
arrangement would increase the spacing required between elements.
Thus the increase in sensitivity per thermocouple pair resulting
from better temperature "capture" would be sacrificed by junction
density reduction. Modeling of this sensor revealed that despite a
significant increase in thermal sensitivity per thermocouple pair,
the density of the junction pairs would be significantly reduced.
In fact, such modeling revealed that although up to 90% of the
temperature difference across the sensor would appear across the
thermocouple pair, the density of junction pairs would be reduced
by a factor of 4 or more. The net gain in sensitivity therefore
would be negligible.
In particular, the results of a modeling analysis of this sensor
showed an increase in .DELTA.T and a decrease in the number of
thermocouple pairs. Various pad sizes and link sizes were modeled
to try to determine the optimum sizes that would produce the
greatest net increase in sensitivity.
The results from each model are shown in the table below with the
third generation heat flux sensor highlighted:
TABLE 1 Gener- Pad Link #/ BotBC TopBC ation # Sz.sup.1 Sz.sup.1
.DELTA.T TC Eff S q" q" 2 1 .times. 1 1 .times. 3 4.085 4 1.0213 q"
q" 3 3 .times. 3 1 .times. 4 12.342 10 1.2342 ##STR1## T q" 2 1
.times. 1 1 .times. 3 4.638 4 1.1595 T q" 3 3 .times. 3 1 .times. 4
13.895 10 1.3823 T T 2 1 .times. 1 1 .times. 3 1.184 4 0.2960 T T 3
3 .times. 3 1 .times. 4 13.200 10 1.3200 T q" 3 1 .times. 3 1
.times. 4 11.150 10 1.1102 T q" 3 1 .times. 3 1 .times. 3 10.233 8
1.2744 T q" 3 1 .times. 2.5 1 .times. 2.5 8.192 6 1.3620 T q" 3 1
.times. 2 1 .times. 2 7.090 6 1.1807 T q" 3 1 .times. 2o.sup.2 1
.times. 2o.sup.2 6.683 6 1.1127 T T 3 1 .times. 3 1 .times. 4
12.961 10 1.2961 T T 3 1 .times. 3 1 .times. 3 12.530 8 1.5663 T T
3 1 .times. 2.5 1 .times. 2.5 11.973 6 1.9955 T T 3 1 .times. 2 1
.times. 2 11.224 6 1.8707 T T 3 1 .times. 2o.sup.2 1 .times.
2o.sup.2 11.247 6 1.8745 T q" 3 1 .times. 4 1 .times. 2 5.359 8
0.6698 T q" 3 1 .times. 4 1 .times. 3 10.386 10 1.0367 T q" 3 1
.times. 4 1 .times. 4 11.694 12 0.9715 T q" 2 1 .times. 1 1 .times.
3 5.024 4 1.2400 T q" 2 1 .times. 1 1 .times. 4 7.933 6 1.2923
.sup.1 1 unit is a 5 mil .times. 5 mil pad .sup.2 0 indicates a
large overlap of the pad and link for the hot junction
In modeling, the boundary conditions on the bottom (BotBC) and top
(TopBC) of the sensor were changed to measure the difference
between its responses to radiative and conductive heat flux. A "q""
boundary condition represented radiative heat flux of 10
watts/cm.sup.2, a "T" boundary condition represented conductive
heat flux between a bottom temperature of 25.degree. C. and a top
temperature of 40.degree. C.
To represent losses in density, the parameter (#/TC) was created.
It is defined as the number of 5 mil.times.5 mil blocks necessary
to make up a repeating pattern. The .DELTA.T was then divided by
the #/TC parameter to determine an effective sensitivity to compare
each of the models. As seen above, none of the models resulted in a
significant increase in the Effective Sensitivity.
It is difficult to relate the effective sensitivities of constant
temperature and constant heat flux boundary conditions. The model
did not compute an overall heat flux for the constant temperature
condition, and the temperatures predicted for the constant heat
flux condition were not equal across the face of the sensor.
4. The 4th Generation Heat Flux Sensor
To overcome the difficulties of the third generation sensor which
use two metals in the middle layer, the fourth generation sensor
was developed. The configuration of the second generation was
modified to improve its thermal performance, while retaining its
high density of thermocouple pairs. With reference to FIG. 6, the
sensor 600 is shown as mounted on a substrate 605. Sensor 600
includes a bottom layer of nickel pads 610 surrounded by polymeric
insulating material 615. The second layer includes a plurality of
nickel links 620. The third layer includes a plurality of copper
links 630. A fourth layer of copper pads 640 is disposed above the
copper links 630. Each of these four layers also includes an amount
of insulating material 615 disposed between the aforementioned
other elements of each layer.
By using such a fourth layer containing copper pads 640, this
configuration provides insulation directly over the "cold"
junctions, effectively isolating the junctions from heat flux,
finite element analysis of this configuration has revealed that the
temperature difference between the "hot" and "cold" junctions
exceeds 75% of the total. Furthermore, such an arrangement can be
produced at the same density as the original first generation
Episensor. Although there is a slight increase in thermal
resistance produced by this fourth layer of copper pads 640, this
arrangement achieves significant advantages over prior heat flux
sensors.
The results from this modelling analysis are tabulated below with
the optimum configuration providing the highest theoretical
sensitivity highlighted. This table shows the effect of the top
layer having various thicknesses.
TABLE 2 Thickness Pad Link (mils) Generation # Sz.sup.1 Sz.sup.1
.DELTA.T #/TC Eff S 0 2 1 .times. 1 1 .times. 3 4.638 4 1.1595 1 3
1 .times. 4 1 .times. 2 6.875 8 0.859 2 3 1 .times. 4 1 .times. 2
6.81 8 0.851 3 3 1 .times. 4 1 .times. 2 6.791 8 0.849 1 3 1
.times. 4 1 .times. 3 12.941 10 1.294 2 3 1 .times. 4 1 .times. 3
12.916 10 1.292 3 3 1 .times. 4 1 .times. 3 12.846 10 1.285 1 3 1
.times. 3 1 .times. 3 14.607 12 1.217 2 3 1 .times. 3 1 .times. 3
14.598 12 1.217 3 3 1 .times. 3 1 .times. 3 14.484 12 1.207 1 2 1
.times. 1 1 .times. 4 16.244 6 2.707 0.5 2 1 .times. 1 1 .times. 4
15.972 6 2.662 1 2 1 .times. 1 1 .times. 4 13.372 6 2.229 1 2 1
.times. 1 1 .times. 3 12.114 4 3.029 ##STR2## 2 2 1 .times. 1 1
.times. 3 12.141 4 3.035 3 2 1 .times. 1 1 .times. 3 11.977 4 2.994
0.5 2 1 .times. 1 1 .times. 3 11.893 4 2.973 0.25 2 1 .times. 1 1
.times. 3 11.482 4 2.871
The end results of these studies predicted a 185% increase in
sensitivity over the first generation sensor design. For 18,900
thermocouple pairs, a Seebek coefficient for the thermocouple of
4.2 .mu.V/ .degree. C., and .DELTA.T of 12.141.degree. C. for an
incident heat flux of 10W/cm.sup.2, the sensitivity should be
approximately 80 mV/(W/cm.sup.2).
5. The 5th Generation (Preferred) Heat Flux Sensor In evaluating
the fourth generation heat flux sensor, the effects of two
different boundary conditions on the top layer were analyzed. A
boundary condition of constant heat flux over the top surface
simulated radiated heat flux, and a constant temperature boundary
condition simulated conducted heat flux. The performance of the
sensor with these two conditions was predicted to be very
different. The sensitivity with constant heat flux was much less
than the sensitivity with constant temperature. With constant heat
flux, a pattern of temperatures developed on the upper face, with
higher temperatures over the polymer insulated portion of the
sensor and lower temperatures over the metal pads. As such, leakage
through the polymer, which has the largest area, was increased,
thereby reducing the effective sensitivity of the sensor to radiant
heat flux.
The solution to this problem was found in adding a thin layer of
high thermal conductivity material over the surface of the sensor,
to prevent development of "hot spots" over the polymer. Such an
arrangement is shown in FIG. 7. The arrangement of FIG. 7 is
identical to the arrangement of FIG. 6, with the exception of the
thin layer of high thermal conductivity material 750 over the
surface of the sensor 700. As such, the fifth generation sensor
700, as mounted on a substrate 705, contains a bottom layer having
nickel pads 710 surrounded by polymeric insulating material 715.
The second layer contains a plurality of nickel links 720, each of
which are surrounded by polymeric insulating material 715. The
third layer contains a plurality of copper links 730 surrounded by
polymeric insulating material 715. A fourth layer of copper pads
740 is disposed above the copper links 730. Although the thin layer
of high thermal conductivity material 750 is preferably made of
aluminum, it is to be understood that other high thermal
conductivity materials could also be used.
The effect of the thickness of layer 750 on sensitivity predicted
by the model is illustrated in FIG. 8. A relatively thin layer of
0.001" (0.003 cm) would have a dramatic and unexpected effect on
the interface temperature. Thinner layers would result in reduced
sensitivity to radiative heat flux, while thicker layers would
retard the sensor's time response without usefully increasing its
sensitivity.
The aluminum layer 750 must be applied in a manner that does not
electrically short the copper top pads of the sensor together. An
electrically insulating adhesive, applied as a thin film, is used
to prevent shorting of the copper top pads.
The importance of the top layer is significant. It is extremely
important to have thermal contact while maintaining electrical
isolation. Without the top layer, the incident heat flux can only
pass through the junctions. Any location that is not a junction
(i.e. dielectric) will merely heat up. This effect of "funneling"
the heat into the junction is the main reason for the increase in
sensitivity. The optimum thickness of the Aluminum layer for the
new design was found to be 2 mils.
In particular, by utilizing an arrangement having a fourth layer
with top copper pads 740 and a thin layer of aluminum 750 to
distribute radiative heat flux over the top surface, numerous
benefits and advantages are achieved. Among these benefits and
advantages are the following:
1. Because both surfaces of the sensor are planar, excellent
conductive heat transfer characteristics are exhibited.
2. The thermal resistance of the sensor is very low, approximately
0.00012 m.sup.2.degree. C./W, because it is thin and based on a
substrate of aluminum and because the paths for heat through the
sensor are mostly metallic.
3. The sensitivity of the sensor is high (approximately 80
mV/(W/cm.sup.2)) because it has a very dense pattern of
thermocouple pairs.
4. The sensitivity of the sensor is also high because a large
proportion of the temperature difference across it is transduced
into a voltage.
5. The response of the sensor to radiation, convection and
conduction is equal. As such, the sensor can be calibrated in one
mode of heat transfer and used for measurements in other modes.
Response to radiated, convected and conducted heat flow are
identical because the temperature distribution across the face of
the sensor is the same in all three cases. Prior heat flux sensors
of this type have had a major deficiency in this regard. With such
prior heat flux sensors, it was not possible to relate their
calibration in one mode to measurements in another.
6. The sensor's sensitivity is the same to heat flowing in either
direction, because the sensing elements are geometrically
symmetrical. This is an advantage in any application where heating
or cooling may occur at different times. For example, in
architectural applications, heat flows in different directions
during different seasons of the year.
7. The sensor is rugged and can be bent over a radius without loss
of continuity or calibration.
8. The symmetry of the sensor guarantees that a zero signal truly
represents zero net heat flow.
9. The conductive aluminum layer on top of the sensor is also a
good electrical shield. Sensor elements are not exposed to
electromagnetic interference, thus the noise induced in the sensor
is very low, and the minimum measurable heat flux is very
small.
10. The aluminum layer on top of the sensor provides some
protection against accidental contact with the copper pads,
eliminating a source of possible error or damage.
11. The aluminum layer provides some mechanical protection from
external elements.
12. Because the sensor is flat and smooth on both sides, the sensor
may be properly bonded to a surface on which it is applied. As
such, the sensor possesses superior bonding characteristics.
Manufacture of the Episensor
With reference to FIGS. 9-13, one method of manufacturing the
episensor of FIG. 8 is shown and will now be described. FIG. 9
shows an anodized aluminum substrate 905 with a first layer of high
thermal resistance photopolymer 910 deposited on it. The
photopolymer is applied on the substrate with uniform thickness,
then exposed through a mask to create four areas which are more
soluble than the surrounding polymer. These areas are dissolved
away chemically, leaving depressions 911, 912, 913 and 914 for the
first layer of nickel ink.
The next step in fabrication of the sensor is to apply the nickel
ink over the top surface 915 in such a way that each depression is
filled, level to the top. The part is then cycled through a high
temperature that causes the ink to solidify within the depression.
The resulting part has a smooth upper surface, ready for the next
layer of photopolymer and ink. These are applied in precisely the
same manner, but with a different mask and pattern.
FIG. 10 shows four nickel pads 916, 917, 918 and 919 produced by
these first steps, as though they were isolated in space. These
pads correspond to pads 710 of FIG. 7.
FIG. 11 shows four nickel links 921, 922, 923 and 924 which are
deposited over pads 916, 917, 918 and 919, respectively, in the
next step of the process. These links 921-924 cover the first four
pads 916-919 and extend beyond them over the polymer surface. Links
921-924 correspond to links 720 of FIG. 7.
With reference to FIG. 12, the first deposited copper metal layer
is shown, with links 931, 932, 933, 934 and 935 completing the
thermopile. Links 931-935 correspond to links 730 of FIG. 7.
FIG. 13 depicts the completed heat flux sensor of FIG. 7, with
copper pads 941, 942, 943, 944, 945 and 946 in a fourth layer.
Copper pads 941-946 correspond to pads 740 of FIG. 7. Constructed
in the manner, the sensor is effectively imbedded in a solid mass
of polymer that is co-planar on one face with the top of these
pads, and fills the entire space down to the aluminum substrate
905. Pads 941 and 946 are included to allow electrical connections
to be made to the ends of the thermopile on the top surface. The
planarity of the sensor gives it a significant advantage in
conductive measurements, because it allows good thermal contact
with the whole surface.
The difference between this fifth generation sensor (FIGS. 7 and
13) and the second generation sensor (FIG. 4) from which it is
derived, is the fourth layer of pads 942, 943, 944, and 945. These
pads create additional thermal isolation for the junctions between
the second and third layers (depicted in FIGS. 11 and 12) in the
form of a layer of photopolymer between those junctions and the top
surface of the sensor. This greatly increases the proportion of
total temperature difference across the sensor that is transduced
into a voltage, nearly tripling its output.
The full benefit of this change is only realized for radiation heat
flux measurements, unless an additional layer of thermally
conductive material 750 covers the top of the completed thermopile.
As previously noted, this layer acts to equalize the temperature on
the top surface and thus prevents a buildup of temperature on the
polymer portion of the top surface, which would increase the
leakage of heat to the lower junctions. Thus, a layer of thermally
conductive material such as aluminum or copper is bonded to the top
surface of the sensor. With this addition, the response of the
sensor to radiation, convection and conduction becomes identical.
No other heat flux sensor is known to have this feature.
The thermally conductive layer 750 must be applied in a manner that
does not electrically short the copper top pads of the sensor
together. An electrically insulating adhesive, applied as a thin
film, is used to prevent shorting of the copper top pads.
Although the above-referenced Ormeply ink process has been
described for manufacturing the heat flux sensor of the instant
invention, it is to be understood that other processes may be used.
For example, high volume applications could utilize techniques in
which the plastic layers are punched out, bonded together and
filled with other types of metallization.
Although the aforementioned preferred embodiment has been shown and
described, it is to be understood that the present invention is to
be defined only by the claims appended hereto.
* * * * *